1. Field of the Invention
The present invention relates to a generating system which employs a fuel cell apparatus.
2. Description of Related Art
FIG. 4 is a schematic drawing of the arrangement of a fuel cell apparatus and peripheral apparatuses for controlling the temperature thereof in a fused carbonate fuel cell generating system, as described in U.S. GRI Report No. FCR-3522-2. In the drawing, a fuel cell apparatus 1 comprises a fuel cell portion la having a fuel gas electrode and an oxidant gas electrode (not shown), a fuel gas passage lb for supplying fuel gas A to the fuel gas electrode and an oxidant gas passage 1c for supplying oxidant gas B to the oxidant gas C electrode. An air supply apparatus 2 recovers power from the gas exhausted from the fuel cell generating system and supplies air D at a high pressure from the outside thereof. A circulating blower 3 for partially circulating the oxidant gas B is provided for controlling the temperature of the fuel cell apparatus 1. The temperature of the gas circulated by the circulating blower 3 on the oxidant gas side is controlled by a heat exchanger 4. Character E denotes a flow of fuel exhaust gas.
The operation is then described below. The fuel cell apparatus 1 has a single fuel cell or a fuel cell stack comprising a plurality of stacked fuel cells each of which produces an electrochemical reaction when the fuel gas A and oxidant gas B are supplied to the fuel gas passage 1b and the oxidant gas passage 1c, respectively. The fuel cell apparatus also serves as an energy transducer for extracting as electrical energy part of the chemical energy possessed by the fuel gas A and transforming the remainder to thermal energy as a by-product.
For example, a molten carbonate fuel cell apparatus is operated at about 650.degree. C. and a phosphoric fuel cell apparatus is operated at about 200.degree. C. The fuel cell apparatuses must be respectively kept at the above operating temperatures by appropriately controlling the temperatures. For example, during a steady-state operation conditions it is necessary to remove the thermal energy produced as a by-product in the fuel cell apparatus 1, i.e., it is necessary to cool the apparatus. On the other hand, during no-load conditions or operation with a small load, it is necessary to reversely heat the apparatus to prevent the fuel cell from getting colder due to heat-loss.
Examples of methods of controlling the temperature of the fuel cell apparatus for cooling or heating it include a method of circulating a liquid-phase heat transfer medium in the fuel cell apparatus 1 and a method of circulating a vapor-phase heat transfer medium. Particularly, the method of circulating the vapor-phase heat transfer medium has an advantage because of its ease of handing and high reliability. Since there is substantially no liquid-phase cooling medium which can be applied to a fuel cell apparatus operated at a high temperature, a vapor-phase heat transfer medium is frequently used in a fuel cell generating system with relatively small output, or a system which employs a fuel cell apparatus operated at a high temperature, e.g., a fused carbonate fuel cell apparatus. FIG. 4 shows an example of a generating system which uses such a vapor-phase heat transfer medium. The oxidant gas B discharged from the fuel cell apparatus 1 is partially recirculated to the inlet of the oxidant gas passage 1c of the fuel cell apparatus 1 by using the circulating blower 3 so that the oxidant gas which is also reaction gas is employed as a heat transfer medium for temperature control. During steady-state load operation conditions the temperature control of the fuel cell apparatus 1 is achieved by cooling the circulated oxidant gas at the heat exchanger 4.
During no-load or small-load operating conditions the temperature control of the fuel cell apparatus 1 is achieved by heating the circulated oxidant gas at the heat exchanger 4.
The conventional fuel cell apparatus configured as described above has the following problems:
(1) Since the vapor-phase heat transfer medium has a small heat capacity, the difference between the temperature of the heat transfer medium gas at the inlet and that at the outlet of the fuel cell apparatus is generally as large as about 100.degree. C. A temperature distribution thus easily occurs in the direction of flow of the heat transfer medium gas. This accelerates the deterioration in characteristics of the fuel cell. PA1 (2) Although the heat required for controlling the temperature of the fuel cell apparatus is received as sensible heat in the fuel cell apparatus or extracted therefrom, the sensible heat is easily lost, for example, as heat radiation from the exhaust pipe, during the temperature control process. This causes a decrease of the energy conversion efficiency in the generating system. Particularly, gaseous sensible heat has a low energy density, and thus the size of the heat exchanger for utilizing exhaust heat increases. It is therefore difficult to effectively utilize the exhaust heat. PA1 (3) When the reaction gas is used as a heat transfer medium, it is necessary to supply the reaction gas in a large amount, for example, which is 3 times as much as the amount required only for electrochemical operation. In the case, the evaporation loss of electrolyte contained in the fuel cell apparatus is accelerated. Since the supply of the reaction gas thus interferes with the control of the temperature of the fuel cell apparatus, the control method is complicated, and the life of the fuel cell is decreased. There is also the problem that the generating efficiency is decreased by an increase in the blower power accompanying an increase in the circulating amount.